This week, science commemorates a remarkable anniversary: the unravelling, 50 years ago, of the structure of deoxyribonucleic acid, DNA, the golden molecule from which our genes are made and which directs the construction of our bodies' cells. Shown by Francis Crick and Jim Watson to be a tiny double helix, a spiral staircase of life-controlling chemicals, DNA has become an icon of our era, an emblem of mankind's mastery over nature. Understanding its behaviour has allowed us to complete the human genome project, create Dolly the Sheep, develop new wonder drugs and unravel the roots of cancer and heart disease.

Yet these achievements, although stunning, were, in a sense, inevitable. Once it was appreciated how DNA splits apart and directs the manufacture of proteins in our cells, researchers were propelled, almost automatically, to these biological goals. Each new act of understanding led, inexorably, to the next magnificent breakthrough.

Not every development was so pre-ordained, however. Some uses of DNA have caused real surprise, even to scientists, and of these, its forensic application in exposing long-forgotten events, in exploiting genes as messengers from the past, has triggered most fascination, and disquiet, as Weinberg makes dramatically clear in her fastidious, absorbing true-life murder story.

Her subject, Helena Greenwood, was an ambitious English DNA researcher working for a Californian biotechnology firm, when she was sexually assaulted at gunpoint in her home. Local businessman Paul Frediani's fingerprints were found nearby and he was arraigned for trial. Helena agreed to give evidence.

She never got the chance. On 22 August 1985, her husband Roger, returning from work, discovered Helena strangled, 'her face, arms and legs covered in cuts, her tights torn, her wallet and other valuables strewn around,' says Weinberg. As investigating officer David Decker put it: 'Helena put up a hell of fight.' Yet there was no evidence, other than motivation, to link Frediani, the chief suspect, to the crime. Police collected fingernail clippings and other evidence and waited while Frediani fought, and lost, his battle against conviction for Helena's rape.

After a few years in jail, he was released, though science continued to move on. Alec Jeffreys invented genetic fingerprinting and used it to pin point the killer of Leicestershire teenagers Dawn Ashworth and Lynda Mann; Kary Mullis created genetic amplification by which billionfold samples of DNA pieces can be grown. It became possible to identify people from only a few scraps of material. Lawyers argued over laboratory accuracy, civil rights activists protested, rightly, that prosecution officials, who control DNA's collection and analysis, were acquiring unfair powers. As a result of such confusion, all DNA evidence was thrown out of the 1995 murder trial of OJ Simpson, letting him walk free.

Still technology improved, to the extent that DNA analysers now require technicians merely to pop in samples to create a person's digitalised genetic barcode. In April 1998, Laura Heilig of San Diego's sheriff's office decided to take another look at the Greenwood case. She examined the fingernail clippings collected by Decker. There was nothing to see, but as Weinberg says: 'That didn't mean that invisible clues were not there, microscopic helixes displaying the identity of Helena Greenwood's killer.' She sent them off for analysis, which revealed the presence of foreign DNA. This was then compared with Frediani's: a perfect match.

In January 2001, Frediani, who had stoutly proclaimed his innocence, was found guilty by a unanimous jury vote. Told of the verdict by phone, Helena's father Sydney, stricken with cancer, murmured a few words and died.

Today, Frediani is serving a life sentence in Corcoran jail after being trapped by the very substance on which his victim had staked her professional future. It is, as Weinberg says, an alluring and ironic symmetry. If nothing else, this story, which Weinberg relates with clarity and flair, gives proper human perspective to our DNA anniversary celebrations. A murderer who would otherwise have escaped justice is now in jail.

But it would be wrong to dwell too much on the collective misery generated by this extraordinary case. (Both Greenwood's and Frediani's families suffered horribly, as Weinberg is at pains to note.) Instead, we should remember an even more satisfying aspect of DNA science: those individuals now relishing freedom after DNA evidence released them from unjust incarceration. As Weinberg points out: 'In April 2002, the one hundredth person was freed from Death Row after DNA had proved him innocent.' And that is an event that is truly worthy of celebration this week.